Thin film transistors (TFTs) are commonly used to drive pixel electrodes of liquid crystal elements of liquid crystal displays (LCDs). A TFT typically includes an active region comprising amorphous silicon, polysilicon or CdSe. Active regions formed of hydrogenated amorphous silicon (a-Si:H) have particular advantages for mass production and production of large-screen displays. Typical goals in the design of an a-Si:H TFT include increasing the field-effect mobility and reducing off current under backlight illumination. However, since a-Si:H typically has a high photo-conductivity, high levels of leakage current may occur under backlight conditions. The leakage current can be especially large in a projection display using high-intensity backlighting. Thus, it is particularly desirable to decrease photo leakage current in a-Si:H TFT in order to obtain a high quality TFT-LCD.
A conventional method for reducing the photo leakage current by reducing the thickness of an undoped a-Si:H layer is described in an article entitled "A 33 cm-Diagonal High-Resolution Multi-color TFT-LCD with Fully Self-Aligned a-Si:HTFTs," by N.Hirano et al, IDRC 94, pp. 369-371 (1994). Though photo leakage current can be slightly lowered by this method, this approach may also decrease field-effect mobility.